This can be cured ervin. You do have a clean disk? Call of Duty simply copies compressed files to harddisk. Go to the file C:\Call of Duty\Main\pak0.PK3 or wherever it is stored on your disk.
Often it does not pass a redundancy check within the program.
Open it with the well known program WinRAR. If not known for any supposed reason then download it. Select pak0.PK3 in its screen and make sure you have done that. Next pick "Repair Archive" from the Commands menu. I suggest the zip option activated. Repair the archive pak0.PK3. Then play. Have fun.